The long-term reliability and safety of implantable cardiac leads is a significant issue. Anomalies of conductors in implantable medical devices constitute a major cause of morbidity. Representative examples of such medical devices include, but are not limited to, pacemakers, vagal nerve stimulators, pain stimulators, neurostimulators, and implantable cardioverter defibrillators (ICDs). For example, early diagnosis of ICD lead conductor anomalies is important to reduce morbidity and/or mortality from loss of pacing, inappropriate ICD shocks, and/or ineffective treatment of ventricular tachycardia or fibrillation (ventricular fibrillation). The early diagnosis of conductor anomalies for implantable cardiac leads is a critically important step in reducing these issues and making ICDs safer.
Multilumen ICD defibrillation electrodes or leads include one or more high-voltage conductors and one or more pace-sense conductors. The leads can be implanted as subcutaneous or intravascular leads. Insulation failures have been known to result in functional failure of the corresponding conductor. Functional failure of a pace-sense conductor may result in symptoms caused by loss of pacing functions for bradycardia, cardiac resynchronization, or antitachycardia pacing. Functional failure of a high-voltage conductor may result in fatal failure of cardioversion or defibrillation.
Thus, one major goal is high sensitivity of diagnosis: identification of lead insulation failures at the subclinical stage, before they present as a clinical problem. A second major goal is high specificity: a false positive provisional clinical diagnosis of lead insulation failure may trigger patient anxiety and lead to potentially-avoidable diagnostic testing. A false positive clinical diagnosis of insulation failure results in unnecessary lead replacement, with corresponding expense and surgical risk.
Insulation failures occur most commonly at three regions along the course of a pacemaker or ICD lead. The first region is within the pocket, caused either by abrasion of the lead insulation by pressure from the housing (“CAN”) of the pulse generator or twisting of the lead within the pocket. The second region is that between the clavicle and first rib, where the lead is subject to “clavicular crush.” The third region is the intracardiac region near the tricuspid valve. This third region is a particularly common site of insulation failure for the St. Jude Riata® lead which is subject to “inside-out” insulation failure due to motion of the internal cables relative to the outer insulation.
It is extremely difficult to detect and localize lead insulation failures on an ICD implanted in the chest of a patient. The taking of x-rays has been attempted to easily identify anomalies but has had extremely limited success, and essentially zero success where the anomaly is a lead insulation failure. For example, FIG. 1 is an x-ray of an implanted ICD, the arrow showing a small area of insulation stress from a tight suture (taken from Radiography of Cardiac Conduction Devices: A Comprehensive Review by Amanda L. Aguilera, M D, Yulia V. Volokhina, D O and Kendra L. Fisher, M D, October 2011 RadioGraphics, 31, 1669-1682). Whether this area of insulation stress has already led to an insulation failure or not is impossible to discern from this image.
Due to the failure of x-ray diagnosis, the primary method in the prior art for monitoring pacemaker and ICD lead integrity is periodic measurement of electrical resistance, usually referred to as “impedance monitoring.” Impedance monitoring uses single pulses. Various methods well-known in the art provide a measure of impedance close to the direct-current resistance.
However, another common issue is that insulation failures commonly present clinically without detected changes in impedance as measured by presently used methods. There are several possible explanations. One explanation is that the range of impedance in normally functioning leads may be wide. For example, it has been reported that high-voltage impedance in normally functioning high-voltage leads may fall approximately 30% from maximum measured values (Gunderson B D, Ellenbogen K A, Sachanandani H, Wohl B N, Kendall K T, Swerdlow C D. Lower impedance threshold provides earlier warning for high voltage lead fractures. Heart Rhythm 8:S19, 2011). Similarly, the range of impedance for pace-sense leads can vary widely. A second explanation is that impedance is determined primarily by body tissue, so that even if an in-pocket insulation failure is present, a test pulse delivered from housing to the affected electrode may not detect the insulation failure unless dielectric breakdown is complete.
The difficulty in detecting an insulation failure with present electrical testing may be appreciated from this example. Consider a fracture in the conductor leading to the SVC coil (SVC conductor). Such a fracture—in its initial stages—may have an impedance (to the body core) of 2 kΩ or more. The typical SVC coil has an impedance on the order of 60Ω Thus the parallel combination of the normal impedance and the “leakage” impedance (from the fracture) would result in an impedance reduction of 1.8Ω which is far lower than the typical (5-10Ω) serial impedance changes seen chronically. Similar difficulties are seen with insulation failures on pace/sense conductors as the tip and ring impedances change significantly with fibrosis and other chronic effects.
In the circuit being measured, most of the resistance is at the electrode-tissue interface of the high-resistance tip electrode, and variations of up to 10% in this value are common. Each individual pace-sense conductor (for example, the conductor to the tip electrode or the ring electrode) contributes less than 10% to the measured resistance. In some ICD leads, this value is less than 3%. Thus even if the resistance in a single conductor doubled or tripled, the overall measured resistance will remain within the expected range. Measurements indicate that resistance exceeds the expected range until the conductor has lost most of its structural integrity. Thus, resistance remains within the expected range even when only a fraction of the conductor is intact. For this reason, resistance measurements are insensitive to partial loss of conductor integrity. Further, resistance measurements have limited specificity. A single, out-of-range value may be an artifact, and marked increases can occur at the electrode-myocardial interface.
In addition to limited sensitivity, present methods for diagnosing lead conductor anomalies have limited specificity resulting in false positive diagnostics. Evaluation of false positive diagnostics adds cost and work to medical care and may contribute to patient anxiety. If a false-positive diagnostic is not diagnosed correctly, patients may be subject to unnecessary surgical lead replacement with its corresponding risks. In the only report on this subject, 23% of leads extracted for the clinical diagnosis of lead fracture tested normally after explant.
Any clinical method for detecting conductor anomalies in implanted leads must make measurements while the conductor and lead are in the body. Typically, the measuring circuit includes the conductor-tissue interface in the body. Thus the measured values will depend both on the behavior of the conductor being evaluated and the conductor-tissue interface.
Existing technology for diagnosis of conductor anomalies in an implantable medical device is believed to have significant limitations and shortcomings. What is desired are method and apparatus that could analyze and identify implantable cardiac lead conductor anomalies at the subclinical stage, before they present as a clinical problem, and do so with a high sensitivity and specificity that minimizes false positives for implantable cardiac lead conductor anomalies.